We are studying the nicotinic acetylcholine receptor (AChR) and its role in the disease myasthenia gravis. AChR is isolated and purified from rat and human skeletal muscle. In some cases AChR is purified from denervated skeletal muscle and is of the extrajunctional type that can be biochemically differentiated from the AChR found at the innervated neuromuscular junction. Using sera from a population of approximately seventy patients treated at the myasthenia gravis clinic, Department of Neurology, University of Alabama in Birmingham, we are characterizing the general properties of the interaction of anti-AChR antibodies and purified muscle AChR. These antibodies are of the IgG type and are believed to be the autoimmune site of attack in myasthenia. We have determined that AChR will typically be bound by a small number (1-4) of anti-AChR antibodies from a given sera, and that the antibodies bind to at least three receptor subunits. We have found that the conventional assay for anti-AChR antibody used in the diagnosis of myasthenia fails to recognize antibodies that bind to the AChR ligand site, resulting in the underestimation of the anti-AChR antibody titer by as much as 100 fold.